The new work, led by researchers at Simon Fraser University in collaboration with the European Space Agency (ESA), responds to concerns raised after a 2020 International Space Station mission detected an unexpected blood clot in a female astronaut's jugular vein. Historically, most space health research has involved male participants, but as more women join astronaut corps and crews, agencies are seeking sex specific data on physiological risks and countermeasures.
In the ESA sponsored VIVALDI I dry immersion study, 18 healthy female volunteers spent five consecutive days in continuous simulated microgravity. Participants lay in a specially designed water tank separated from the water by a waterproof sheet, allowing them to float while remaining dry and mimicking the fluid shifts and unloading of body weight seen in orbit. The team used rotational thromboelastometry, or ROTEM, to track coagulation dynamics in real time during and after immersion.
ROTEM measurements showed that coagulation time, the period before clotting begins, became longer in the simulated microgravity environment. Once clotting started, however, clots formed more quickly, and the final clots were stronger and more stable, indicating they were more resistant to breakdown. Menstrual hormone levels, measured alongside the coagulation tests, did not influence these clotting changes in the participants.
Senior author Andrew Blaber, a professor of biomedical physiology and kinesiology at SFU, notes that this pattern of slower onset but faster and stronger clot formation appears in healthy women under short term simulated microgravity. He explains that the findings do not indicate an immediate danger over five days, but they do highlight the need to understand how these alterations might evolve over months in orbit, when crews are far from emergency care and evacuation options.
Lead author Tiffany Stead emphasizes that the altered clotting response becomes more concerning when combined with how blood redistributes in the absence of gravity. On Earth, gravity pulls blood toward the legs, and most dangerous clots typically form in deep veins of the lower limbs, giving more time for the body or clinicians to act before fragments reach critical organs. In space, fluids shift toward the upper body and head, changing where clots are likely to appear.
Previous observations have shown that in microgravity, clots are more likely to develop in the jugular vein in the neck. From that location, a dislodged clot has a short path to the lungs or heart, increasing the risk of pulmonary embolism, heart attack, or stroke. Blaber says this proximity means that the same clotting behavior that is benign in the short term on Earth could have more serious implications in orbit, where diagnostic resources and treatments are limited.
The study, published in the journal Acta Astronautica, advances efforts to define sex specific cardiovascular and hematological risks of spaceflight. By focusing on women in a controlled dry immersion protocol, the researchers were able to isolate microgravity like effects and rule out menstrual hormone fluctuations as a driver of the observed changes in coagulation. The work adds a critical piece to the broader picture of how microgravity influences vascular health.
Blaber's team is now analyzing comparable dry immersion data from male participants to determine whether similar clotting alterations occur in men and how any differences might shape future medical guidelines. These results, together with operational experience from orbit, are expected to inform individualized monitoring strategies and countermeasures for mixed gender crews on missions that may last six months or longer.
Space agencies have already adapted some procedures in response to clotting events observed in astronauts. Crews on the International Space Station now perform regular ultrasound examinations of the jugular veins during missions, using the same technology that first revealed a clot incidentally in 2020. Increased surveillance aims to detect developing clots before they can cause life threatening events and to guide timely intervention in the confined environment of a spacecraft.
The SFU Aerospace Physiology Laboratory works closely with the Canadian Space Agency and international partners to study how spaceflight and spaceflight analogs affect multiple physiological systems in astronauts. Ongoing and planned projects examine cardiovascular regulation, fluid balance, and orthostatic intolerance during and after missions, with the goal of protecting crew health on future lunar and Mars expeditions. The clotting data from VIVALDI I are expected to feed into this broader research portfolio.
By identifying that women in simulated microgravity develop clots that start later but grow faster and become stronger, the VIVALDI I study underscores that sex and environment both matter in predicting vascular risk beyond Earth. As agencies design longer and more distant missions, incorporating such findings into screening, training, and on orbit medical support will be critical to keep astronauts safe while they live and work away from gravity.
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Simon Fraser University
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